Photovoltaic generation is subject to long-time or short-time fluctuates in the amount of generated power depending on the amount of solar irradiation, and stable power supply is difficult as compared with a conventional power generation system such as thermal power generation. A synchronous generator used in thermal power generation has the potential to prevent fluctuations in a system frequency, and has a speed governor to adjust generated power when a system frequency fluctuates, stabilizing a system frequency. On the other hand, a power generation system combining a photovoltaic generation module and an electric power storage unit is not provided with such a frequency adjustment function as the one provided in a synchronous generator to prevent fluctuations in a system frequency. Therefore, if a number of such power generators is installed in a power system, stabilization of a system frequency will become difficult.
There has been proposed a power generation system which maintains constant total effective power of a photovoltaic generation module and storage battery constant, or a method of preventing a short-time fluctuation in the output of a photovoltaic generation module, by combining an electric power storage unit represented by a storage battery and a photovoltaic generation module.